1. Field of the Invention
The present invention relates to a metal bump and a method of fabricating thereof, and more particularly, to a metal bump with an insulating sidewall and a method of fabricating thereof.
2. Description of the Related Art
The attachment of a bared die to a glass panel (called COG:chip on glass) is one advanced application for electrically connecting integrated circuits (ICs) achieving lighter weight, smaller size, lower cost and less power consumption demanded in various display products. Consequently, COG has been successfully adopted for small products (less than 4 in.), such as display panels for telephones and copiers, which have one or two chips, medium-size products (4˜11 in.), such as video cameras and navigation systems, which require 3˜12 chips, and large products (more than 11 in.) for notebook PCs.
The quality and reliability of the liquid crystal display (LCD) module depends on the way in which the driver IC is attached to the glass panel. Anisotropic conductive film (ACF) is the most popular material for attaching the chip to the glass panel. ACF is an adhesive film consisting of dispersed, microscopic, electrically conductive particles 3-15 μm in diameter and an insulating adhesive film 15˜35 μm thick. Various kinds of conductive particles, such as carbon fiber, metal (Ni, solder), and metal (Ni/Au)-coated plastic balls have been proposed, and the uniformity of the conductive particles distribution is considered an influence on the electrical property and reliability of ACF. Also, various types of adhesive materials, such as thermoplastic, thermosetting, and mixed thermoplastic and thermosetting materials have been proposed. In general, ACF is classified into two types. One has conductive particles 5 μm in diameter covered with a very thin insulating layer, wherein the thin insulating layer is broken when the particles are deformed, the bared conductive particles serving as a bridge for electrically connecting the metal bump on the chip and the bonding pad on the glass panel. However, the breaking of the conductive particles during the fabricating process cannot be ensured; therefore, there is no guarantee of effective contact between the metal bump and the bonding pad. The other type of ACF is a double-layer type, which consists of one layer filled with conductive particles 3 μm in diameter and the other layer with no conductive particles, so that the functions of conduction and adhesion are separated. This can ensure the effective contact between the metal bump and the bonding pad. Nevertheless, when too many conductive particles exist in the space between two adjacent metal bumps, a lateral connection between the two adjacent metal bumps is easily formed, resulting in an electrical short.
Please refer to FIG. 1A to FIG. 1C. FIG. 1A is a top view of the layout of a glass substrate 10 according to the prior art. FIG. 1B is a top view of the layout of the predetermined area 15 shown in FIG. 1A. FIG. 1C is a top view of the layout of a chip 20 according to the prior art. A glass substrate 10 of the LCD module comprises a first area 12 for disposing an array of thin film transistors (TFTs), a second area 14 for disposing data IC chips on the predetermined areas 15, and a third area 16 for disposing scan IC chips on the predetermined areas 15. Each predetermined area 15 comprises a plurality of first bonding pads 18. A chip 20 which is the data IC chip or the scan IC chip comprises a plurality of second bonding pads 22, wherein each second bonding pad 22 corresponds in position to each first bonding pad 18.
Please refer to FIG. 2A to FIG. 2D. FIG. 2A to FIG. 2D are schematic cross-sectional diagrams of a method of connecting the chip 20 and the glass substrate 10 according to the prior art. As shown in FIG. 2A, a schematic cross-sectional diagram along the line 2—2 shown in FIG. 1B, an ACF 24 is attached to the surface of the glass substrate 10 to cover the first bonding pad 18. As shown in FIG. 2B, a schematic cross-sectional diagram along the line 2′—2′ shown in FIG. 1C, a metal bump 26 is fabricated on the second bonding pad 22 of the chip 20. As shown in FIG. 2C, the surface of the chip 20 is downwardly placed on the predetermined area 15 of the glass substrate 10, wherein each metal bump 26 corresponds to a first bonding pad 18 of the glass substrate 10. By means of the adhesion of the ACF 24 and the downwardly exerted pressure, the chip 20 is tightly attached to the glass substrate 10. A thermal process is then performed to cure the ACF 24. Therefore, the conductive particles 25 sandwiched between the top of the metal bump 26 and the surface of the first bonding pad 18 serve as an electrically connecting bridge. However, as shown in FIG. 2D, the distribution of the conductive particles 25 cannot be controlled in processing, and thereby many conductive particles 25 that exist between adjacent metal bumps 26 may laterally connect with each other to cause electrical shorts. Especially when the size of the metal bump 26 is incorrectly designed or the alignment between the metal bump 26 and the first bonding pad 18 is inaccurate, the conductive particles 25 are more easily laterally connected in the narrow distance between the two adjacent metal bumps 26. This will significantly decrease the functioning and reliability of the LCD module.